18. - 20. 5. 2011, Brno, Czech Republic, EU

PROPERTIES OF PLASMA NITRIDED LAYERS IN DEEP CAVITIES

Zdeněk POKORNÝa, Vojtěch HRUBÝa, Karel STRÁNSKÝb, Jaromír KADLECa

a University of Defence, Department of Mechanical Engineering, Kounicova 65, Brno, Czech Republic

b University of Technology, Faculty of Mechanical Engineering, Technická 2896/2, Brno, Czech Republic

Abstract

The nitriding behaviour of heat-treated alloyed carbon steel was carried out on two samples with different diameters. The influence of microstructure and nitriding conditions on the thickness and hardness of nitrided layer were investigated. Bored cylindrical samples were quenched and subsequently tempered. Plasma nitriding was performed at 500°C for 6 hours with m ixture of H2 and N2 in plasma nitriding appliance. Microstructure and mechanical properties of nitrided layers were characterized using optical spectrometry, optical microscopy, microhardness testing and SEM with energy dispersive analyser. Plasma nitriding process increased the surface hardness of samples by 100%. The nitrided depth was also estimated using cross-sectional microhardness profiles. The results show that plasma nitriding creates a nitrided layer consisted of compound layer and nitrogen diffusion layer. Behaviour of Nht thickness was experimentally obtained in dependence on length of cavity. The influence of nitrogen content on microhardness and Nht thickness of nitrided layer was proved.

Keywords: plasma nitriding, cavity, nitrided layer, nitrogen, length of nitriding.

1. INTRODUCTION

The carbon alloy steels have been generally used in automobile industry for their high hardness and strength. Low corrosion resistance sometimes limits their industrial application. Plasma nitriding process is a chemical-heat treatment widely used to increase the surface hardness, fatigue strength, wear and corrosion resistance of steels [1, 2]. Parameters of plasma nitriding layer are not dependent only on process parameters of nitriding, such as duration, temperature, pressure, voltage and nitrogen potential, but it is also dependent upon the substrate alloy type and in this case of selected alloy on their microstructure [1-3]. The aim of the present investigation was to study the creating of nitrided layer inside the cavity with the diameter of 6 and 8 mm. This study deals with mechanical properties of nitriding layers inside the cavities. Chemical composition of steel was verified for selected chemical elements by GDOES/Bulk method on LECO SA 2000 spectrometer. Microstructure was evaluated by laser confocal microscope Olympus OLS 3000 [1]. Thickness and microhardness of plasma nitrided layers were measured by microhardness testing method in accordance with DIN 50190 standard [5] on automatic microhardness tester LECO LM 247 AT. The local composition was found on energy dispersive semi-quantitative analyzer Phillips (EDS) EDAX 9900.

2. EXPERIMENTAL

Bars of 32CrMoV12-10 steel in untreated state were bored with diameters of 6 and 8 mm. Samples of length 500 mm were heat treated in accordance with Tab. 1.

Fig. 1. Image of cut off sample

Tab. 1. Temperatures of heat-treated steels

Procedure Temperature [°C]

Oil quenching 940

Salt tempering 650

length

18. - 20. 5. 2011, Brno, Czech Republic, EU

A microhardness of treated material of samples was 600 HV0,05. Plasma nitriding was carried out in PN 60/60 RÜBIG appliance according to Tab. 2. The charge was consisted of 2 cylindrical samples (cavities) which were plasma nitrided at the pressure of 500 Pa for 6 hours.

After plasma nitriding process, the samples with the diameters of 6 and 8 mm were cut off. The length of the first sample was 30 mm, each other was 12 mm. The lengths of the samples were folowing: 30, 42, 54, 66, 78, 90, 102, 114, 126, 138, 150, 162, 174, 186, 198, 210, 222, 234, 246, 258 mm. All samples were wet grounded using silicon carbide paper with grit from 80 down to 2000 and subsequently polished. Created nitrided layers are documented in Fig. 2.

Chemical composition of material was measured by GDOES/Bulk method (Tab. 3). The depth profiles were evaluated by GDOES/QDP method and are shown in Fig. 5. Glow discharge optical spectroscopy (GDOES) measurements were performed in LECO SA-2000, with argon glow discharge plasma excitation source, calibration of nitrogen: JK41-1N and NSC4A standards.

Confocal laser microscope LEXT OLS 3000 with outstanding resolution of 0.12 µm and magnification range from 120x to 12400x were used for structure documentation. Microhardness was measured by Vickers microhardness method on automatic microhardness tester LM 247 AT LECO at 50 g load and 10 s dwell time. The major Vickers microhardness numbers were derived from five measurements such an average value according to Fig. 3. The result of five measurements and the cross-section structure of chemical etched material is shown in Fig. 4.

Tab. 3. Chemical composition of 32rMoV12-10 steel

C Mn Si Cr Mo V P S

GDOES/Bulk

0,30 0,47 0,25 2,95 0,89 0,28 0,002 0,001

DIN standard

0,30

0,35

<

0,60

<

0,35

2,80

3,20

0,80

1,20

0,25

0,35

<

0,025

<

0,010

Tab. 2. Parameters of plasma nitriding process

Temperature [oC] 500

Duration [h] 6

Gas flow H2/N2 [l.min-1] 24/8

Bias [V] 530

Pressure [mbar] 5

Pulse length [µs] 100

Fig. 2. The chemically etched confocal cross-sectional structure of tempered steel

Fig. 3. Real image of measured sample from LM 247 AT

18. - 20. 5. 2011, Brno, Czech Republic, EU

Following equation was used for calculation of Nht thickness X (1) in accordance with DIN 50190 standard:

X = [[Y * 0.1] *10] + 50. (1) where, X is Nht thickness in mm, Y is the average microhardness number from five indentation´s

patterns in HV 0.05 [kg]. The local chemical compositions of plasma nitrided layers in length of cavities were observed by SEM method in combination with energy dispersive micro analyser PHILIPS EDAX 9900.

Fig. 4. Microhardness depth profile of plasma nitrided layer in cross-sectional structure

Fig. 5. Depth profiles of 32CrMoV12-10 steel

Fig. 6. The thickness of nitrided layers in dependence on content of nitrogen at pressure 500 Pa in cavity with diameter of 6 mm

Tab. 4. Local chemical composition measured on SEM

Length [mm]

Nht thickness

[mm]

Nitrogen

[%wt]

30 0,12 7,22

42 0,12 6,15

54 0,11 6,05

66 0,09 5,26

78 0,09 4,99

90 0,09 5,82

102 0,08 4,21

114 0,06 4,24

126 0,00 1,75

Depth [µm]

Ana

lyte

wei

ght p

erce

nt

18. - 20. 5. 2011, Brno, Czech Republic, EU

Measurement of nitrogen content was performed from two local spaces with magnification 25x. The results from analysis are shown in Tab. 4 and Fig. 6.

3. RESULTS

Heat-treated samples with diameters of 6 and 8 mm were plasma nitrided at pressure 5 mbar. Microhardness of plasma nitrided layers confirmed increasing of surface microhardness about 500 HV0,05 which is shown in Fig. 4. Nht thickness of plasma nitrided layer was measured in accordance with DIN 50190 standard (1). The significant part of results is shown in Tab. 5. The cavity with diameter of 6 mm was nitrided to the length 126 mm, the last value of Nht thickness was measured in length 114 mm. In case of cavity with diameter of 8 mm the nitrided layer was presented along all length of cavity. The values of Nht thickness decreased to 0.03 mm in length 270 mm and then increased back to higher values because all samples were nitrided from both sides.

4. CONCLUSION

Experiment showed that plasma nitriding process is applicable not only for flat surfaces but for cavities too. These created nitrided layers improve corrosion resistance and mechanical properties of steel. It was proven that the diameter of cavity has remarkable influence on the plasma nitriding in length in accordance with Fig. 7. Peaks which established during plasma nitriding process (Fig. 7) have connection with plasma density and concentration of nitrogen. Fig. 6 shows that concentration of nitrogen has significant influence to the thickness of both nitrided layers. Compound layer, diffusion layer and nitrogen concentration are interdependent. Created plasma nitrided layer improves corrosion resistance and mechanical properties.

ACKNOWLEDGEMENTS

The work was supported by a research project of the Ministry of Defence of the Czech Republic

„Support of education and science in engineering“, and by Specific research 2011.

Tab. 5. Results of microhardness measurements

Length [mm]

Diameter of

8 mm 6 mm

Nht thickness [mm]

30 0,18 0,12

114 0,14 0,06

150 0,11

0,00

246 0,07

258 0,04

270 0,03

282 0,04

284 0,05

Fig. 7. Course of microhardness in cavities with diameter 6 and 8 mm

18. - 20. 5. 2011, Brno, Czech Republic, EU

REFERENCES.

[1.] JOSKA, Z., et al. Characteristics of Duplex Coating on Austenitic Stainless Steel. Key Engineering Materials, 2011, vol. 465, no. 1, p. 255-258.

[2.] KADLEC J., DVORAK M.: Duplex surface treatment of stainless steel X12CrNi 18 8. In Strength of Materials. 2008, Vol. 40, No. 1, p. 118 - 121.

[3.] JOSKA, Z., et al: Chemicke listy, 2010, Vol. 104, p. 322-325.

[4.] POKORNY, Z, HRUBY, V. Plasma Nitriding of deep narrow Cavities. In Key Engineering materials, vol. 465, 2011. pp. 267-270, ISSN 1013-9826.

[5.] DIN 50190-2: Hardness depth of heat-treated parts; determination of the effective depth of hardening after flame or induction hardening